According to IATA. website, going to Vietnam, both Canadian and US passport holder requires to have six month validity on their passport. Not sure about other Nationality. You can try the IATA. website for the passport that you use.
Just to share my experience only, I know that for a fact that Thailand, Japan, South Korea, US, and Hong Kong don't require your passport to be valid for six months if you are holding either a Canadian or US passport. Some of the embassy website have incorrect info. Cheers